Mission Critical Fuel - What is Diesel Bug?
Understanding Microbial Contamination
Diesel Bug: The Hidden Threat in Stored Diesel
Diesel fuel stored long term is susceptible to microbial contamination, caused by a mix of bacteria, yeast, and fungi that can grow in storage tanks. This contamination is often referred to as “Diesel Bug”.
Diesel Bug can damage tanks, fuel lines, pumps, and injectors, reduce fuel economy, and increase emissions. For businesses relying on standby or emergency generators, the most serious risk is blocked fuel filters and unexpected engine shutdowns, especially following a mains power outage.
What Causes Microbial Contamination?
Microbial growth requires three key elements, often called the “eternal triangle”:
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Water – Even small amounts of water in a fuel tank create a habitat for microbes.
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Nutrients – Trace organic compounds in diesel or from contamination provide food for microbial growth.
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Favourable Temperature – Microbes thrive in moderate temperatures, typically between 10°C and 40°C.
When these three factors are present, microbes multiply, producing acids and sludge that damage fuel systems.
Most microorganisms enter a tank while travelling down the fuel supply chain, moving from tank to tank. These microorganisms, commonly referred to as “diesel bug”, use the fuel itself along with fuel additives as a food source. Once water is present in the tank, it provides the environment necessary for their survival. When all three elements of the eternal triangle are present, microbial growth (biomass) is inevitable.
Standby generators are typically test-run for only 30 minutes per month. During such short runs, the amount of contamination passing into the primary filter sight bowl is often too small to notice. Problems arise when the engine is required to run for extended periods. During these longer runs, significantly more contamination is drawn through with the fuel. It is at this critical stage that filters can become blocked, potentially leading to engine shutdown.
Sample on the right contains rusty water with a small amount of fuel sitting on top.
H₂O and the Problems It Causes
When water accumulates in fuel storage tanks, it creates an ideal environment for microbial growth. The sample on the right shows rusty water with a small layer of fuel sitting on top.
While water ingress and severe microbial infections are not uncommon, they usually occur due to inaction or a lack of preventative measures. Even a small amount of visible contamination in primary filter sight glasses or water collection bowls typically indicates that microbial growth has already taken hold at the bottom of the tank. Microbial activity can begin from just tiny water droplets, so it is critical to keep water out of fuel tanks.
In standby fuel storage systems, water can enter tanks in several ways: condensation, leaking seals, or contaminated fuel supplies.
Condensation occurs when tanks “breathe” moist air, allowing water to condense on the walls. Temperature differences between the inside and outside of a tank — for example, overnight or after an engine has run and hot fuel is returned to the tank — can also cause condensation. The more vacant wall space in the tank, the more water can accumulate.
External sources of water such as rain, flooding, or wash-down water can enter tanks through leaking fill points, broken pipework, old seals, or poorly located tank breathers. Underground tanks are particularly vulnerable to this type of ingress.
Water in supplied fuel is another risk. While fuel suppliers generally maintain high standards, contamination can occasionally occur, allowing water and microbes to enter your tanks unknowingly.
Once water enters the tank, it can separate from the fuel, run down the walls, and settle at the bottom as free water. Although some free water may be removed by a primary fuel filter, fuel pickups are usually positioned above the tank floor, leaving water undisturbed for long periods. This creates a perfect environment for microbial growth and biomass/sludge formation.
Beyond promoting microbial growth, water also causes corrosion in tanks and fuel system components. Flat-bottom tanks are particularly prone to heavy rust scaling.
Best Practice Procedures
A robust Fuel Management Programme should include a number of best practice procedures to reduce the risk of water ingress and microbial growth. These include:
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Keep tanks above 80% full wherever possible. Regular top-ups prevent condensation from forming on tank walls.
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Drain water and sediment regularly using drain valves or dedicated low points in the tank. Even small amounts of water should be removed frequently.
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Inspect tank components regularly, including seals, pipework, man-ways (inspection hatches), and breathers. Repair or replace any damaged or deteriorated parts promptly.
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Source fuel from reputable suppliers. For large deliveries, request a sample before accepting the fuel.
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Install moisture traps on tank breathers. These prevent moisture from being drawn into the tank as air moves in and out.
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Sample and test fuel regularly, particularly from the bottom of the tank, to monitor water content and microbial activity.
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Consider an automated fuel polishing system for tanks with high volumes or critical applications.
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Incorporate a Fuel Management Programme into your preventive maintenance plan (PPM) to ensure consistent monitoring and treatment.
Fuel Phase – When cloudy as shown emulsified/suspended water is also present in the fuel.
Interface – The bulk of the microbes (bugs) live at the interface between the fuel and the water.
Free Water Phase – Always at the bottom as water is heavier then fuel. There can be up to a 1000 times more bugs in the water then in the fuel itself.
Microorganisms in Diesel Fuel
There are over 100 different microbes, including bacteria, yeasts, and moulds, that can grow in middle distillate fuels such as diesel and aviation fuel. Of these, around 30 species are capable of causing problems in diesel, with a few being particularly troublesome. Growth rates vary depending on conditions in the tank, making diesel especially vulnerable to contamination during prolonged storage. This is why the standby and emergency power generation industry is often most affected by diesel microbial contamination.
Bacteria, yeast, and fungi primarily grow in the free water at the bottom of the tank, usually visible at the interface between water and fuel. Microbes can also colonize fuel throughout the reticulation system and even downstream of filters.
While microbes feed on hydrocarbons (alkanes) in the fuel, it is the presence of water that enables their growth. Fuel additives can also provide additional nutrients, further promoting microbial activity. Visible contamination is most noticeable at the fuel-water interface when free water accumulates, but microbial growth can also occur on tiny suspended water droplets dispersed throughout the fuel. In these droplets, the droplet surface itself acts as the interface where microbes thrive.
In modern High Pressure Common Rail Diesel (HPCRD) engines, it is often the suspended water and microbes that cause the most significant damage. Even with correct filtration, these engines, which have extremely tight tolerances, are highly sensitive to contamination. Microbes carried in water droplets can harm lift pumps, injectors, and other critical components, leading to reduced performance or expensive repairs.
What is “Diesel Bug”?
The most problematic microbe in diesel fuel is the fungal organism Hormoconis resinae (H. resinae), previously known as Cladosporium resinae. This organism is commonly referred to as “Diesel Bug” in diesel applications, and as the “Jet Fuel Fungus” in aviation.
H. resinae is particularly damaging because it can adhere to tank surfaces and promote corrosion. It can attack both steel and alloy tanks, sometimes causing pitting corrosion by penetrating the metal substrate. This microorganism is extremely resilient, able to survive freezing and thawing cycles as well as very dry conditions, making it one of the toughest and most persistent contaminants in fuel storage systems.
An example of corrosion in a stainless tank.
How Microbial Material Affects Fuel Systems
Microbial material can be drawn up through the fuel pick-up lines and deposited in the primary filter. By the time it becomes visible, tens of thousands or even hundreds of thousands of spores have combined to form what appears as black spots, stringy matter, or watery sludge in filter sight bowls.
If this microbial material passes beyond the filters, it can cause extensive damage to fuel pumps and injectors, resulting in costly repairs. While proper filtration should prevent this, it is remarkable how these tiny, resilient microbes can still create significant problems in fuel systems.
This black and dark organic material is a combination of bacteria, yeast and the H.res fungi.
Early Stages of Microbial Infection
At the start of an infection, Hormoconis resinae is highly aggressive because it can produce large amounts of biomass. As the infection develops, other fungi such as Yarrowia lipolytica, a common yeast, begin to take over by competing for the same food sources. This typically causes H. resinae to die off, leaving the dead material either floating in the fuel or adhering to tank surfaces, often with significant damage hidden beneath.
Some microbes can hold water in suspension while others have the ability to produce water themselves.
Fuel Management Programmes
Fuel Management Programmes, including regular monitoring of fuel quality, should be implemented for all tanks supporting Mission Critical Equipment. Sampling and testing from the bottom of the tank on a scheduled basis provides the best opportunity to detect free water and microbial activity. For larger bulk tanks, sampling from the top and middle of the tank should also be considered.
While chemical testing for suspended water and other key components can confirm that the fuel meets government regulations, only a full microbial laboratory test can determine if the fuel is truly fit for purpose.
If you have any questions, require specific advice on your tank systems, or would like Pacific Fuel to implement a Fuel Management Programme, please do not hesitate to contact us.